1. Field of the Invention
The present invention concerns a mechanical apparatus of the type wherein a tool mounted on a reciprocating piston is used to deform a work piece. In a preferred embodiment, the present invention concerns a hydraulic piercing and stripping assembly in which hydraulic power is provided to a streamlined piercing unit for both piercing (i.e., work stroke) and stripping (i.e., return stroke), without resorting to compressed gas and without requiring complex or precise machining of either the power unit or the piercing unit.
2. Description of the Related Art
A feature common to all piercing presses is a repetitive reciprocating stroke motion, wherein a pierce stroke is followed by a stripping stroke. The pierce stroke requires considerable power in order to pierce a metal work piece. The stripping stroke must also be performed under some amount of power in order to ensure that the work tool does not remain stuck in the work piece and is completely withdrawn from the work piece.
Generally, such presses are driven by a drive force generating unit which may be, e.g., a mechanical crank and flywheel arrangement, an electric motor powered pump (see, e.g., U.S. Pat. No. 3,756,065), an electric motor powered pump connected to a hydraulic system, etc.
The press-driven tool module can be thought of as being comprised of two main components: (1) a power transmitting unit (power unit), and (2) a work unit (e.g., piercing unit). The power transmitting unit conveys the drive force from the drive force generating unit to the piercing unit. Power transmission may be by mechanical linkage, but for greater flexibility of the piercing unit power transmission is preferably by hydraulic means. In the piercing unit, the drive force is converted to a piercing stroke, usually by the action of hydraulic pressure against a work piston, which acts on a piercing ram to which a piercing tool is attached.
As discussed above, during the stripping stroke the piercing unit must be returned to the starting position under power. Various arrangements have been developed for providing this power, but each is characterized by certain inherent disadvantages.
One common means for providing return power is to connect a helical coil spring directly to the work ram. For example, U.S. Pat. No. 3,939,686 teaches a hydraulic pressure powered shear where the cutting blades are retracted by means of springs attached directly to the cutting blades. U.S. Pat. No. 3,168,918 teaches a crimping machine wherein the hydraulic ram is withdrawn under the action of a return spring. U.S. Pat. No. 5,065,609 teaches a device wherein a pressing piston is retracted directly by means of a helical coil mechanical spring. A disadvantage of these arrangements is that the helical coil spring takes up space. Space in the working area is limited, and the presence of an external spring may interfere with the range of mobility of the piercing unit in the working area. It is thus usually desirable to have the piercing unit constructed so as to be as streamlined as possible.
Alternatively, the piercing unit may be operated based upon a reciprocating piston defining a piercing chamber and a stripping chamber within a cylinder, with hydraulic power being supplied for both piercing and stripping. Stripping power may be continuous low level power, with piercing carried out against the force of the pressurized medium in the stripping cylinder space. Alternatively, piercing and stripping power may be alternated, with a piercing valve opening and a stripping valve closing to permit pressurization of the piercing chamber and a piercing stroke, and when the piercing stroke is completed, the piercing valve closing and the stripping valve opening for pressurization of the stripping chamber causing a stripping stroke.
For example, U.S. Pat. No. 5,271,262 teaches an apparatus for forging a large caliber ring. The power stroke is accomplished by hydraulic pressure, and a return cylinder device is powered by a hydraulic pressure supply source. U.S. Pat. No. 4,941,342 teaches a multi-ram forging system employing two-way rams, with hydraulic forging pressure driving the rams in the working movement, and with return being accomplished by return hydraulic pressure in a return piston chamber. Each of these systems requires a complex system of pressure accumulators, pressure regulators, control mechanisms, couplings and fittings.
PCT/DE/00370 attempts to streamline a press-driven cross-piercing or bending unit by incorporating a pressurized gas cylinder within the piercing unit, the pressurized gas providing spring power for return of the piercing unit to the starting position. More specifically, the unit comprises a work unit (including a work cylinder and a work ram slidably mounted in a horizontal cylindrical bore, the work ram adapted for receiving a work tool), a power unit (including a hydraulic power cylinder and a vertically displaceable power unit piston and piston rod mounted in the hydraulic power cylinder), the power unit being driven by a drive force generating unit (which preferably includes a flywheel and crank connected to a press ram, the press ram disposed for acting upon a power unit piston and piston rod), with a hydraulic pressure line communicating between the power cylinder and the work cylinder. The press ram acting on the hydraulic power cylinder piston rod and piston under pressure causes transmission of hydraulic fluid under pressure through the hydraulic lines to the work cylinder. Since the work unit is connected to the power unit by means of flexible high pressure lines, the work unit can be oriented and positioned independently of the power unit. This design also makes it possible to operate a multiplicity of working units via a single power unit at the same time. Further, the operation of the press-driven tool module is exceptionally simple since the hydraulic medium is essentially supplied into the power unit without any pressurization, and is essentially placed under high pressure for only short periods of time when acted upon by the drive force generating unit.
Although this design provides a number of advantages, there are also certain inherent features which are less than optimal. For example, one disadvantage of this type of hydraulic piercing unit is the need for complex and precise machining of the gas cylinder which provides the return (stripping) power. Further, the added pressurized gas system is liable to problems such as contamination, worn seals, loss of pressure over time, need for recharging, need for lubrication, etc. Further yet, the amount of return force which can safely be provided by a pressurized gas system is limited, and for certain applications compressed gas will not safely provide the required return forces.
It is an object of the present invention to provide a hydraulic piercing unit wherein a significant amount of power can be provided in the return stroke, e.g., for insuring that a mechanical piercing or bending tool is completely withdrawn from the work piece after the work operation.
A further object of the invention is to provide a hydraulic power return unit wherein no separate complex return power generating unit is required, i.e., no separate motor or accumulator or control system.
A further object of the invention is to provide a hydraulic piercing unit wherein the piercing unit can be positioned and oriented independently of the power transmitting unit.
It is an object of the invention to provide a device in which the stripping mechanism is simple in construction. The stripping mechanism should use the same hydraulic power as the piercing mechanism. The stripping mechanism should not employ compressed gas.
Finally, it is an object of the invention to provide a piercing unit which efficiently utilizes power from the drive force generating unit to power the return stroke.